Field of the Invention
The invention relates to a homogenization device for homogenization of a magnetic field with a non-magnetic carrier and compensation elements which formed at least partially of a magnetic material, the carrier having a carrier wall and the carrier wall surrounding a carrier interior. In the homogenization device located in the magnetic field, the magnetic field penetrates into the carrier interior through a first carrier region of the carrier wall and emerges from the carrier interior through a second carrier region of the carrier wall and each of the compensation elements which are located on the carrier contribute to the homogenization of the magnetic field at least in the carrier interior.
Description of Related Art
Homogenization devices of the initially mentioned type can be used in various applications for homogenization of a magnetic field. Only one exemplary application of the homogenization devices is the homogenization of the magnetic field of a nuclear magnetic flowmeter. Nuclear magnetic flowmeters determine the flow rate of a medium which is flowing through a measuring tube from the nuclear magnetic resonance measurements which have been taken on the medium.
Nuclear magnetic resonance measurements require media with elements whose atomic nuclei have a magnetic moment. This is given in atomic nuclei with a nuclear spin. The nuclear spin can be understood as an angular momentum which can be described by a vector, and accordingly, the magnetic moment can also be described by a vector which is aligned parallel to the vector of the angular momentum. In the presence of a magnetic field, the vector of the magnetic moment of an atomic nucleus is aligned parallel to the vector of the magnetic field at the location of the atomic nucleus. Here, the vector of the magnetic moment of the atomic nucleus precesses by the vector of the macroscopic magnetic field at the location of the atomic nucleus. The frequency of the precession is called the Larmor angular frequency ωL and is proportional to the amount of the magnetic flux density B. The Larmor frequency is computed according to the formula ωL=γB, in which γ is the gyromagnetic ratio which is maximum for hydrogen nuclei and γ=276.5·106 rad/(sT).
Nuclear magnetic resonance measurement methods excite the atomic nuclei of a medium which have a magnetic moment in the presence of a magnetic field and measure the action of the excitation. In nuclear magnetic flowmeters, the flow rate of the medium through the measuring tube is determined using the measured action of the excitation. An excitation of the atomic nuclei causes an in-phase precession of the atomic nuclei with the Larmor angular frequency ωL of the atomic nuclei which are precessing beforehand with statistically distributed phases to one another and the excitation perturbs the previously prevailing equilibrium state of the precessing atomic nuclei. The perturbation, as long as the precession is in-phase, is measurable as a macroscopic alternating magnetic field, the angular frequency of the alternating magnetic field being the Larmor angular frequency.
To perform the nuclear magnetic resonance measurements, nuclear magnetic flowmeters have a magnetic field generating apparatus for generating the magnetic field in the medium which is flowing through the measuring tube. Here, the conventional magnetic flux density in the flowing medium is B≈0.3 T. A fluctuation of the magnetic flux density in the medium on the order of magnitude of the magnetic flux density of the terrestrial magnetic field, for example, by ΔB=30 μT, leads to a fluctuation of the Larmor frequency by ΔωL=276.5·106 rad(sT)·30 μT≈8.1·103 l/s of the precessing atomic nuclei which are flowing in the medium. The fluctuation of the Larmor frequency itself leads to a deterioration of measurement accuracy and also causes a loss of the in-phase precession of the atomic nuclei, as a result of which the amplitude of the macroscopic alternating magnetic field decreases; this leads to a further deterioration of measurement accuracy.
Conventional magnetic field generating apparatus of nuclear magnetic flowmeters are built out of permanent magnets, the permanent magnets generally being arranged as Halbach arrays. But, generally the required measurement accuracy of nuclear magnetic flowmeters does not result from the homogeneity of a magnetic field which has been produced in this way, for which reason it is necessary to homogenize the magnetic field by a homogenization device. However, magnetic fields which have been produced with electromagnets often do not meet the demands on the homogeneity of the magnetic field which are imposed by the application.
A homogenization device of the initially described type in particular for homogenization of the magnetic field which has been generated by the magnetic field generation apparatus of a nuclear magnetic flowmeter can be located in the medium flowing through the measuring tube in the magnetic field such that the measuring tube lies in the interior of the carrier of the homogenization device in accordance with the invention and is surrounded by the carrier wall of the measuring tube. The magnetic field of the magnetic field generation apparatus penetrates into the carrier interior through the first carrier region of the carrier wall and emerges from the carrier interior through the second carrier region of the carrier wall. The compensation elements located on the carrier homogenize the magnetic field in doing so at least in the carrier interior and thus also in the medium.
The carrier of a homogenization device is formed of non-magnetic materials. Non-magnetic materials are wherein they influence a magnetic field, if at all, only to a degree which is negligibly small for the respective application. Diamagnetic and paramagnetic materials are also regarded as non-magnetic materials. In contrast, the compensation elements consist at least partially of a magnetic material. Magnetic materials are ferromagnetic, ferrimagnetic and antiferromagnetic materials. It is common to them that they influence a magnetic field.
In the prior art, a homogenization device of the initially described type is known in which each of the compensation elements is arranged on the carrier by cementing. The disadvantages of this homogenization device are apparent in the examination of the process of homogenization of a magnetic field which encompasses several steps.
In a first step, the magnetic field is measured and usually several of the compensation elements are arranged on the carrier at certain positions according to the measurement results. In a second step, the magnetic field with homogenization devices which are located in the magnetic field is measured in the carrier interior. In this second step, the magnetic field in the carrier interior is usually more homogenous than without the homogenization device, but the homogeneity of the magnetic field often still does not meet the demands of the application. Consequently, a third step is necessary in which compensation elements arranged according to the measurement results from the previous step are removed or shifted. In a fourth step, the magnetic field with the homogenization device located in the magnetic field is again measured in the carrier interior. In this fourth step, the magnetic field in the carrier interior is usually more homogeneous than after the first step. If the homogeneity of the magnetic field meets the demands, the process of homogenization is completed, otherwise the third and fourth step are repeated until the homogeneity meets the demands of the application. Accordingly, the homogenization of the magnetic field is an iterative process.
In the homogenization device known in the prior art, to remove and shift each individual one of the compensation elements, the cementing must be dissolved and the residues of the cement must be removed. If a cement is used whose cementing performance is low for simple removal of the compensation elements, the operating reliability of the cementing is adversely affected and additional measures must be taken to fix the compensation elements which have been cemented tight. If an adhesive is used whose adhesive performance is high for ensuring the operating reliability of the cementing, damage to the carrier is possible when the compensation elements are removed. Moreover, the reproducibility of the arrangement of the compensation elements at predetermined positions is subject to disruptively large inaccuracies.